Boreholes created into the earth for extraction of mineral deposits such as oil and natural gas pass through numerous and varied geologic formations. These geologic formations have varied chemical compositions, permeabilities, porosities, pore fluids, internal (pore) pressures, and material properties. Important material properties that significantly impact well construction operations include compressive strength, tensile strength, fracture initiation pressure, fracture propagation pressure, porosity, Young's (elastic) modulus, Poisson ratio and bulk modulus.
Wide contrasts in formation pressures, formation material properties, and formation fluid types often require isolation and treatment of certain geologic formations. Isolation and treatment may be required to treat a weak formation, to increase near-wellbore fracture initiation pressure, to consolidate weak zones, to cure lost circulation, to reduce formation permeability, to seal off flow zones, to isolate high/low pressure zones, to shut off undesirable water or gas supply, to address damage to the tubing caused by collapse in formation and subsequent damage to pipes, or as a shut off plug for wells which are to be shut off permanently or temporarily, or as a so-called “kick-off” plug to prepare a site for the drilling of a new well from the remaining upper section of a former well. Also, in the drilling of multiple branched wells there is often a need to reinforce and seal the transition zones of the surrounding formation. These transition zones are subject to large mechanical stresses. In addition, there may be weak formations in wells that have been drilled such that they deviate substantially from a vertical position, or where part of the well is horizontal.
Weak formations may result where, for example, the fracture initiation pressure of one formation may be lower than the internal pore pressure of another formation. The increased pressure in a borehole created by penetrating one formation may cause a lower strength formation to fracture. Similarly, the fluid pressure gradient in a borehole required to contain formation pore pressure during drilling may exceed the fracture pressure of another, weaker formation exposed in a borehole.
Attempts may be made to isolate specific formations and reinforce them with steel casing, or with cement or other treatments known in the art. Where steel casings are cemented in a borehole to isolate geologic formations having significantly different properties, each of these casing strings is costly and results in a reduction in the diameter of the borehole in subsequent sections as the borehole is deepened. It is desirable, therefore, to minimize the number of casing strings required to reach the desired depth.
It is known in the art to use cement to line boreholes, however a disadvantage of cement is that the curing step may require up to 24 hours, which is a disproportionately long period of time to wait, especially when the production site is a very costly offshore operation. A further disadvantage of cement is that in view of its particle based structure the material exhibits relatively poor penetration capabilities in formations, which may result in reduced sealing effect.
There are references in the literature using resin based cementing materials for geothermal wellbores. In “New, Novel Well-Cementing Polymer Concrete Composite”, American Concrete Institute (ACI), Special Publication 69: “Application of Polymer Concrete”, 1981, part 69-5, pp. 73-92, Zelding, A. N., et al describe a system based on resin with initiator and inhibitor, where complete curing of the resin depends on the presence of water. The main component of the resin is organic siloxanes.
WO 94/12445 discloses an alternative material to cement and water slurries, developed for the completion of primary and secondary cementing of deep, hot oil wells with static background temperature in the range of 120-200° C. (248-392° F.). The binder is based on diallyl phthalate resin with the setting/curing time controlled by the addition of a temperature sensitive peroxide initiator and a suitable inhibitor.
Various sealants are known in the art for lining or strengthening boreholes. Where sealants are employed, a resin or monomer must be selected for each well that is compatible with the drilling/completion fluid used. Epoxy resins provide the best comprehensive strength, tensile strength and adhesion properties. However, epoxy resins and/or their curing agents have poor compatibility and poor performance with olefins, esters, and paraffinic hydrocarbon fluid.
WO 97/15746 discloses a composition for sealing different types of zones in oil wells, which includes a monomer, initiator, inhibitor and optional filler as well as other additives.
U.S. Pat. No. 4,556,109 discloses a system based on curing according to a condensation mechanism, rather than free radical polymerization.
Acrylate or methacrylate resins/monomers are available that are soluble in olefin, ester, and paraffinic hydrocarbon fluids. However, alone, these monomers and resins fail to provide the type of material properties required for geosynthetic composite linings. They typically have poor tensile strength, poor fracture toughness, and low compressive strength. Blends of acrylate monomers containing prepolymers have improved tensile strength, compressive strength, and fracture toughness. However, the prepolymers used in these blends are often insoluble in the hydrocarbon fluids previously discussed.
There is a distinct need in the art for chemical formulations which can provide in-situ geosynthetic composites which are completely soluble in non-aqueous drilling fluids, invert emulsion drilling fluids, drilling fluids made with diesel and mineral oils, and combinations thereof, and blends of any of these with olefins, esters, and paraffinic oils. There is also a need for in-situ geosynthetic composites which provide good radial penetration and which set faster.
It would constitute a distinct advance in the art if a formulation were available for forming a composite in-situ that contained a blend of components that could be adapted to provide optimum material properties depending on the properties and conditions encountered in the formation by simply altering the type and concentration of crosslinking monomer and/or solvent and by varying the amount of each component. It would be additionally advantageous if the formulation exhibited improved compatibility with any combination of non-aqueous drilling fluids, invert emulsion drilling fluids, or drilling fluids made with diesel and mineral oils, and any blend of any of said drilling fluids with one or more esters, olefins, and paraffin oils, or combinations thereof.